A. Field of the Invention
The present invention relates to optical fibers, and an improved process and apparatus for production of the same. More particularly, the present invention relates to optical fibers having good optical properties and improved apparatus and batch process for production of the same.
B. Description of the Related Art
In the production of Large Core Polymeric Optical Fibers (LCPOF), two processes are known for such production: (1) batch and (2) continuous. The present invention is described using the batch process that uses claddings to essentially provide a mold for the light transmitting core. During the batch process, the claddings are filled with highly purified monomer mixtures and placed inside a reactor where polymerization of the monomer mixture occurs. Claddings are usually made of materials which are rather expensive and at times difficult to handle. Therefore, there is a desire to reduce cladding costs through thinner cladding walls as well as using other cladding arrangements and cladding materials.
The present batch methods of production can cause imperfections at the core-clad interface when the core becomes separated from the cladding during and after polymerization. Furthermore, voids and bubbles may form in the light transmitting core during formation. Separation of the light transmitting core from the cladding as well as the formation of voids and bubbles reduce light transmission and are undesirable.
With reference to FIG. 1, in order to avoid voids and imperfections that may occur during the polymerization reaction the cladding 20 wall is made relatively thick in the prior art. The reason is that the polymerizing mass that becomes the core 22 exerts an inward force onto the interior surface of the cladding 20--a pseudo-adhesion--during the reaction. Most monomer mixtures shrink extensively during polymerization, exerting a force inward onto the cladding 20 that they are attached to, in most cases a fluoropolymer tubing. The cladding is preferably made from fluoropolymers that inherently possess a lower refractive index. Such fluoropolymers offer many advantages such as chemical inertness, smooth (highly glossy) surface, high heat resistance and relatively low refractive indices among other attributes. Nonetheless, fluoropolymers are relatively expensive ($0.03 to 0.04/gr.), and, due to inward force exerted by the polymerizing mass, have had to be used in relatively thick wall thicknesses to retain shape (roundness). Substantial cost savings can be realized if the fluoropolymer claddings wall thickness can be extensively minimized (less than 0.1 mm) while retaining the other positive attributes. Nonetheless, fluoropolymer claddings with such thin walls, even when encased in a relatively thick manufacturing jacket, deform during the core manufacturing. The latter results from the pressure exerted inward during the polymerization of the monomer mixtures (shrinkage of the polymerizing mass).
If the force pulling inward is greater than the structural force of the cladding 20 or the force holding the cladding to the manufacturing jacket 24, then the cladding 20 is separated from the manufacturing jacket 24 and deformed--the radial uniformity along the optic is thereafter compromised. This phenomenon can be noted as de-lamination of fluoropolymer cladding 20 away from the manufacturing jacket 24, and in short "de-lamination", see FIG. 1.
Another cause of de-lamination is where a cladding arrangement is collapsed due to the external pressure experienced from the loading of claddings on top of each other as adversely encountered in some manufacturing operations. The pressure exerted from the upper claddings on the lower claddings force the lower claddings to deform, and when the upper claddings are lifted, the fluoropolymer cladding separates from the manufacturing jacket causing de-lamination.
A way to resolve the de-lamination problem is to introduce means to create a force inside the cladding, such as pressurizing the polymerizing mass inside the cladding. In addition, such pressurization is useful in reducing the number of voids and bubbles in the light transmitting core. However, there is a limit to how much pressure can be exerted onto the polymerizing mass due in part to the strength of the cladding material in which over pressurization results in bursts in, or other damage to, the cladding. Similarly, under pressurization may result in separation of the cladding from the manufacturing jacket.
The limits on the amount of pressure applied to the core, imposed by the strength of the cladding, are undesirable. The cladding strength can be improved by the use of a manufacturing jacket in which the cladding is surrounded by a jacket that is made from a stronger material than the cladding to bolster the cladding walls under higher pressures. However, the manufacturing jacket increases the cost of materials to manufacture the LCPOF and must be removed following formation of the light transmitting core. Thus, there is a need to create optics under relatively higher pressures where the optic core is made relatively free of bubbles and voids and the core-cladding interface is essentially defect free to maximize light transmission. Furthermore, it is desirable to reduce the manufacturing costs by conserving the amount of cladding material used and eliminating or reducing the need for manufacturing jackets.